1. Field of the Invention
The present invention relates to a probe for signal detection having a rod-shaped nano structure attached thereto and a method for manufacturing the same, and more particularly, to a probe for detection of a surface signal or a chemical signal having a rod-shaped nano structure, such as tungsten nanowire, carbon nanotube, boron nanotube, etc., which is attached to an tip portion thereof and a method for manufacturing the same.
2. Description of the Related Art
Until recently, the nano world of atom or molecular unit was an unknown field as being too minute to be observed even by a microscope of a high resolution. With introduction of an SPM (scanning probe microscope) in 1980s, however, the nano world finally has become structurally identified. The first kind of atomic microscope such as the SPM was an STM (scanning tunneling microscope) while the most commonly used one is an AFM (atomic force microscope).
FIG. 1 shows a construction of the AFM in general. As shown in FIG. 1, the AFM has a tapered tip 10 of a pyramid shape formed at one end of a cantilever 12, a tiny rod (100 μm×10 μm×1 μm), which is produced by micromachining. When the tip 10 is approached to a surface of a sample 14, interactions (repulsion or attraction) occur between the tip 10 and atoms on the surface of the sample 14. The interactions comprise mainly Van Der Waals force and are of about a nano Newton level or less (10−9 N). Because of such interactions, the cantilever 12 is bent or shows a change in resonance frequency when it moves over the surface of the sample 14. Thus, it is possible to determine a geometrical morphology of the sample by measuring the bend and the change in resonance frequency. Meanwhile, the bend and the change in resonance frequency of the cantilever 12 can be measured by using a laser 16 and a photodiode 18. At this stage, a feedback control is used to continuously keep the measurement on the surface, whereby a stage 20 having the cantilever 12 attached at its end can continuously measure the bend of the cantilever 12 while maintaining a uniform distance between the tip and the sample. Thus obtained results are analyzed to acquire surface information on the sample.
The AFM is used as a fundamental research equipment to measure or observe the nano level. The AFM is also used in various fields as process equipments for production at the nano level. The processing technologies using the AFM such as soft probe lithography or scanning probe lithography (SPL) are under intensive research and study recent days.
The most fundamental core technology of the AFM resides in the probe tip. The image resolution and reproducibility of the AFM are determined according to the shape and size of the probe tip.
In general, tip of the cantilever of the AFM is formed to have a pyramidically tapered shape. However, carbon nanotube (CNT) is recently attracting public attention because of its abundant advantageous characteristics. The CNT is attached to a tip of a pyramid to be used as a probe.
The tip of the AFM is advantageously made of a material atomically having a high aspect ratio and a high resilience. Seen from this perspective, the CNT tips are known to have ideal characteristics to improve performance of the AFM in terms of measurement, operation and production, e.g., excellent sharpness, a high aspect ratio, mechanical stiffness and resilience as well as readiness in adjustment of chemical components. In addition, the CNT tips have advantages in that they have a long life span and are preferably used to measure a deep and narrow-width structure. The CNT tips have a resolution as high as 1 nm or less.
However, it is very difficult to individually form a high quality carbon nanotube in a desired shape at a desired position. The conventional methods such as laser ablation or arc discharge serve to form a nanotube like an entangled skein of thread. It is very difficult to purify, separate and manipulate such an entangled nanotube so as to be attached to a single device.
For instance, Oshima et. al. disclosed in U.S. Pat. No. 5,482,601 a method of vapor depositing carbon nanotube by means of arc discharge, while Mandeville et al. disclosed in U.S. Pat. No. 5,500,200 a method of massively producing MWNT by using catalyst.
Even though such methods are effective for developing a new complex material by massively producing the carbon nanotube or carbon fibril, it is almost impossible to separate individual nanotube and precisely attach each one to a desired position, as stated above. Thus, it is inappropriate to mount a nanotube tip on the probe of the AFM as a commercial method.
Recently, Cheung et. al. developed a method of directly growing MWNT or SWNT by coating catalyst on a microgroove, which was manufactured on a silicon substrate by means of chemical vapor deposition (CVD) (Carbon Nanotube Tips Direct Growth by Chemical Vapor Deposition, PNAS, Chin Li Cheung et. al., Vol. 97, No. 8). According to this method, catalyst particles are coated on a silicon substrate so as to individually grow a probe tip of the AFM. Thereafter, a carbon nanotube is grown by using carbonic oxide gas of high temperature.
However, it is very difficult to attach catalyst particles to the tip of the silicon pyramid. The SWNT grown at the tip of the pyramid is sized 1 μm˜20 μm. In fact, however, its size should approximately be 30 nm˜100 nm to be attached to the AFM. Although discharging methods are used to reduce the size, they rarely succeed in precisely adjusting the size.
In particular, Dai disclosed in U.S. Pat. No. 6,401,526 a more effective method of manufacturing an AFM tip, to which a nanotube has been attached. According to this method, a liquid phase precursor is coated on the tip of the AFM, and the coated AFM is grown by the CVD method. Discharging process is performed to adjust size of the manufactured nanotube. Here, the liquid phase precursor comprises salts including metals, a long-chain molecular compound, and a solvent. Dai also suggested a method of simultaneously coating the precursor on the tips of a plurality of pyramids by means of micro contacting printing.
Another recently reported method is to coat the precursor on a wafer, onto which a massive amount of silicon pyramid for AFM is mounted, by means of spin coating. The precursor is removed from the wafer except on the pyramid by means of etching. A carbon nanotube is grown in the gas including carbon by means of the CVD method. (Wafer Scale Production of Carbon Nanotube Scanning Probe Tips for Atomic Force Microscopy, Applied Physics Letter, Vol. 80, No. 12, Erhan Yenilmez etc., 2002, March, 00.2225–2227).
However, all of these methods pose a problem in that coating the precursor exactly to a desired amount is very difficult primarily because of mechanical and chemical properties of the precursor.
Meanwhile, Nakayama et. al. disclosed in U.S. Pat. No. 6,528,785 a method for manufacturing an electrode (i.e. nanotube) on a holder by fusion welding. According to this method, a carbon nanotube is first positioned between two electrodes. Then approaching the holder close to the carbon nanotube until they are attached to each other, and by means of electron beam or coating film, the carbon nanotube (CNT) is firmly fastened to the holder.
Although there have been introduced a variety of methods for manufacturing coating films, basically all of the methods are directed to one technique that material for use in coating is not the one being coated. Rather, the coating film is formed by a chemical reaction between a gas-exposed nanotube and a holder.
Unfortunately however, the above coating method driven by a chemical reaction is unrealistic and thus, cannot be succeeded in reality. This is because a microscopically protruded nanotube can also be influenced of the chemical reaction, and the nanotube itself can be damaged during a work process.
Besides the above, the method by Nakayama et. al. has a very low yield, and thus, is not appropriate for mass production. First of all, it is almost impossible to visually confirm whether the carbon nanotube is firmly adhered to the holder. Also, because the manufacturing process is usually conducted on SEM (Scanning probe microscope), it takes a great deal of time. Even then, it only raises concerns about the possibility of nanotube getting damages during the process. Moreover, when a carbon nanotube, one of the nano structures like SWNT (Single Wall NanoTube), gets too small, it is difficult to confirm the carbon nanotube as a SEM, so the process also becomes out of control, making the assembly thereof virtually impossible.